There is often great confusion regarding the use of the words supper and dinner. I shall try to help.
“Dinner” has been used since Roman times to signify the largest and most important meal of the day, no matter what time of day it is eaten, but usually at the noon time or early evening. (Lack of proper lighting at night meant most families went to bed early). The original spelling was desjunare.
“Supper” is a more recent word, derived from the French word souper, meaning a light meal often of soup.
In the United States, the words tend to be used interchangeably. However, this is incorrect. When in doubt, just use dinner. If people ask, just do what I do, and say that it should be dinner; tell them that the last supper was over 2000 years ago, and to commemorate it they had a portrait made.